I *can* work the problem in reverse without error.
1. place an instance of the nested family in the parent family.
2. click it and select Edit Family...
3. Edit the Family
4. Save/overwrite the external nested Family file
5. Load into parent Family, and overwrite family + parameters.
no errors.
Maybe this is actually the intended workflow?
Still not sure why the other method would through an error, though...
I'm pretty intent upon maintaining accurate and updated external nested
family source files (*.rfa); and my first workflow seemed to be the best way
to insure that? Otherwise, once the nested Family is edited through the
parent family, if it's not saved externally and just re-loaded into the
parent Family; then that external source file is not out-dated, and left
behind...